Visit a paintball shop and pick up a used 20 oz or 32 oz canister.
Expect to pay about 50 bucks.
Or visit the forums for paintballers and see if you can get a good deal.
I have a 20 oz tank with the remote but I've been getting frozen mice/rats instead of gassing them lately.
The tank cost me 50 and the remote cable cost 60.
The valve flow control they had in the shop cost 80, but I wasn't going to spend that much extra. If you're careful you can simply open the nozzle on the canister slowly and the CO2 will seep out at a good rate.
Actually, if anyone wants my CO2 tank and remote, I can bring them to the show in September. Will trade for one of Neil Meister's awesome little Uroplatus hatchlings